Yes, most SQL books don't explain why, or what is behind the syntax, but merely give you 'cookbook' approaches to standard problems.

Spend some time at www.dbdebunk.com, and if possible, purchase some books by Chris Date. (Chris (or C.J.) Date is one of the original researchers who developed the relational data theory, along with E.F. Codd, back in the 1970s). At www.dbdebunk.com, Date and a couple others discuss many things about sets, trees, substitutability, etc... and much more info can be found in their books. We had a discussion about some of this the forum a few months ago in The Future of RDBMS.

Unfortunately you must be prepared to find out that these researchers believe SQL was a bad decision from the first, because it doesn't fully implement the possibilities of relational algebra. In fact, there has been virtually no company with guts enough to take this step, with the possible exception of Alphora, with their product called Dataphor. I am going to be evaluating it soon, so I will provide some more information here in the forums, in the next few weeks. I sincerely hope that Dataphor is everything the Alphora people claim, because it is high time the software industry had something better than SQL.

Also, there are many software companies trying to sell 'alternative' solutions to SQL, which usually involve some sort of "Object oriented", "Multivalued", "Post-relational", "XML-based" system. The problem with most of these is that they are usually tied to a particular application framework, such as Java, and they don't fully implement the relational paradigm. This means they can do trees quite well, which is SQL's weak point, but they are not so good for many other generalized operations involving sets, normalization, views, etc...